The information in this document is based on the Cisco IOS® Software
Release 11.1, and later.

The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.

When you start a session on the router, you begin in user mode, which
is often called EXEC mode. You can either issue the
telnet command to the router, or use a console
connection in order to initiate most EXEC mode connections. However, you can
also use a dialup connection to start an EXEC session. In such a scenario, the
dialup connection does not run a framed protocol, for example Point-to-Point
Protocol (PPP), over the link.

The EXEC Callback feature enables the Cisco IOS Software to return a
call to a device that dials in, connects to the EXEC, and requests callback.
Therefore, the client can still maintain an EXEC session with the router, but
can avoid the phone surcharges. In order to enable this feature, use the
service exec-callback global configuration command.
You can use this feature for these reasons:

Consolidation and centralization of phone billing

Cost savings on toll calls

Access control

This document illustrates these two callback scenarios:

Callback with no-verify (typical home user)

A client at a remote site dials in and
authenticates.

The client is disconnected.

The access server calls the remote client, and starts an EXEC
session. (The access server has the callback number
configured.)

Callback to any number with verification (typical mobile
user)

A client at a remote site dials in and
authenticates.

The client enters a telephone number to receive the call
back.

The client is disconnected.

The access server calls the remote client, authenticates and then
starts an EXEC
session.

When you configure the PC modem (callback client), initiate a reverse
Telnet session to the modem of the PC. One way to do so in Windows 9x is to use
HyperTerminal, and select "Connect using: Direct to COMx" (where COMx is the
modem port of the PC) to establish a connection.

Here are sample outputs from the PC HyperTerminal session making the
connection to the router:

PC Client: EXEC Callback With No-verify (HyperTerminal
View)

ats0=1
!--- AT command to set modem to autoanswer mode.
OK
atdt 55555
!--- AT command to dial the modem of the router.
CONNECT
username: austin-01
password:
Callback initiated - line is disconnected
NO CARRIER
RING
CONNECT
maui-nas-04>

PC Client: EXEC Callback to Any Number With Verify
(HyperTerminal View)